With the advent of the wireless technology, lots of products such as set-top-boxes, gateways and smart home devices comprise embedded antennas. The embedded antennas are generally integrated within the product all around a printed circuit board (PCB) supporting at least the wireless chipset. The chipset is connected to the antennas via antenna cables of different lengths.
The integration of these antennas could impair the wireless system performances if they are not properly designed, by picking up noise from different sources of the wireless product such as for example, in a set-top-box, from high speed and/or high power buses (PCi-e, RGMII, Sata, USB, HDMI, . . . ), from a digital chip (CPU), from feeding lines of a SDRAM memory, etc. . . . This noise can couple to the antenna either through the radiating element or through the shielding of the antenna cable due to the common mode currents. These leakages of electric current can happen when the feeding of the dipole antenna is unbalanced.
FIG. 1 shows a schematic view of a dipole antenna fed with a coaxial cable and illustrates the common mode current issue. This dipole is composed of two radiating elements, the first radiating element being connected to the central feeding conductor of the coaxial cable and the second radiating element being connected to the shielding of the coaxial cable. The electric current that comes from the central feeding conductor of the coaxial cable is denoted IA. The electric current that comes from the inner side of the shielding of the coaxial cable is denoted IB where IB=−IA. However, outside of the coaxial cable, this current IB is spread between the second radiating element of the dipole (IB−IC) and the outer side of the coaxial cable (IC). The current flowing on the outer side of the coaxial cable, called common mode current IC, can radiate and couple to external noise sources, which must be avoided in modern wireless systems. Moreover, this unwanted current leakage all along the coaxial cable creates several additional radiating sources that are combined to the radiation of the radiating element. That leads to an increase of the antenna directivity and cross-polarization, and a modification of the radiation pattern shape. Both impacts affect MIMO system performance since in this case the transceiver output power must be reduced in order to comply with regulation specification and the angular coverage is low.
Different solutions have been developed to reduce this parasitic coupling and/or reduce the common mode current IC.
One solution consists in increasing the antenna cable length to find a new cable routing avoiding the coupling with the different noise sources. The major drawback of this solution is that it increases the cable losses and thus provides, with an additional cost, lower antenna efficiency.
Another solution consists in using a balun (contraction of “balanced to unbalanced transformer”) that converts unbalanced signals into balanced signals. The balun is inserted between the cable and the antenna. Several baluns can be used, such as for example folded balun, sleeve balun, split coax balun, half wavelength balun or candelabra balun. This balun may be a ceramic balun and/or use ferrite beads or RF chokes/inductors to prevent the common mode currents returning back down on the outer of the cable. This solution adds extra-cost to the antenna and can modify the radiation pattern shape and/or increase the directivity with interaction between the antenna and the additional devices. The balun can also be integrated to the dipole antenna and realized in a printing technology. In that case, the balun is inserted between the radiating elements of the dipole, which increases the size of the antenna.